1. Field of the Invention
This invention relates generally to riser system arrangements for offshore floating facilities such as floating production, storage, and off loading vessels (FPSOs) and including hybrid riser systems extending from the sea floor to the moored vessel for the transport of hydrocarbon fluids. The invention is particularly directed to riser arrangements or systems for stabilizing the upper ends of simple catenary risers, which are in turn connected by way of relatively more flexible riser elements to a moored vessel or other floating facility.
2. Description of the Prior Art
Prior riser systems have included flexible risers which may or may not be continuations of seabed flowlines, where the risers are interfaced with a floating storage facility such as a FPSO, semi-submersible production vessel, etc. With deep water subsea production systems it is advantageous from a cost perspective to provide a rigid flowline (e.g., steel, etc.) as a riser, yet a means is necessary to decouple vessel motions and induced loads from a rigid pipe system. Typically, a flexible pipe between the rigid flowline and the vessel is used for this purpose. A rigid flowline coupled to a flexible hose-like riser is called a hybrid riser system.
U.S. Pat. No. 5,639,187 discloses a marine riser system which combines rigid steel catenary risers (called SCRs) with flexible "hose-like" pipe or flowlines. The SCRs extend from the sea floor in a gentle catenary path to a large submerged buoy positioned at a depth below the turbulence zone of the sea. Flexible risers are connected to the SCRs at the submerged buoy and extend upwardly to a floating platform or vessel used as a surface production and/or storage and off loading facility.
FIG. 1 illustrates a prior art arrangement which includes a floating vessel 10 such as a Floating Production, Storage and Offloading (FPSO) vessel floating on a sea surface 30 and secured to a seabed 32 by means of anchor legs 16 which substantially prevent rotation of a turret 12 which is rotationally supported on vessel 10. In other words, the vessel is capable of weathervaning about the stationary turret 12 under forces of wind, currents and waves. Steel Catenary Risers (SCR) 14 run from seabed 32 sources of hydrocarbons (not shown) to a Steel Catenary Riser Interface Buoy 18, called a "SCRIB. A flexible riser hose 20, typically suspended in a double catenary configuration, is coupled to each SCR 14 at SCRIB 18. The upper end of each flexible riser 20, runs to the turret 12 and connects to a fluid coupling (i.e., a swivel) and then via a pipe to a vessel holding tank.
In the prior art arrangement of FIG. 1, if the upper ends of SCR's 14 at SCRIBs 18 are not restrained in some way, they are free to move in response to vortex-induced vibrations (VIV) or disparate current effects. In order to decrease the effects of such sea current forces, a prior art arrangement provides a link 22 coupled between the turret 12 (or to an auxiliary device secured to the turret 12) and to the SCRIB 18 for each riser. The link 22 provides substantial stability to a riser, and links 22 for all of the risers provide enhanced stability to the system. The tension load of a link 22 need be only a fraction of the load of the SCR 14 itself, because much of that load is reduced by the SCRIB 18. A system of tension links 22, one for each riser 14, advantageously also prevents fouling of the multiple risers with one another. A constant tension device may be coupled to each top tension link 22 to minimize the magnitude of vessel motions transferred to the risers.